AbstractReactivity ratios for the copolymerization of styrene (M1) with some secondary butyl fumarates (M2) indicate that ther1values are more dependent on the inductive effect of the alkyl groups than on the steric effect and that ther2values approach zero. Copolymerization rates of the secondary dialkyl fumarates with 65 mole‐% styrene were found to be consistent with ther1values and the rate of styrene homopolymerization. However, mono(1‐methylpropyl) fumarate copolymerized with styrene at a rate more than twice that of styrene homopolymerization; this agrees very well with the modified copolymer equation which is obtained when the terms of the general copolymerization equation which containr2are set equal to z